The invention is concerned with the catalytic cracking of sulfur-containing hydrocarbon feedstock in a manner such as to effect a significant decrease in the emission of noxious oxides of sulfur in the gases emitted from the regeneration zone of a fluid catalytic cracking (FCC) unit. Carbon monoxide emissions may also be reduced.
A FCC unit is composed of three sections: cracking, regeneration and separation. The cracking reactions take place continuously in the reactor at temperatures between about 900.degree. and 1000.degree. F. The spent catalyst is often stripped with steam and continuously regenerated at about 1100.degree. to 1400.degree. F., and then recycled to the reactor. The cracked hydrocarbon products are finally separated, e.g., in a fractionation system.
The cracking of hydrocarbons that takes place during their contact with the catalyst results in carbonaceous deposits (coke) on the catalyst. Additionally, some sulfur, originally present in the feed hydrocarbons is also deposited on the catalyst. It has been reported that approximately 50% of the feed sulfur is converted to H.sub.2 S in the FCC reactor, 40% remains in the liquid products and about 4 to 10% is deposited on the catalyst. These amounts vary with the type of feed, rate of hydrocarbon recycle, steam stripping rate, the type of catalyst, reactor temperature, etc.
Coke deposits tend to deactivate cracking catalyst. Cracking catalyst is advantageously continuously regenerated to low coke levels, typically below about 0.5% by weight, to perform satisfactorily before it can be recycled to the reactor. Carbonaceous material is typically removed from the catalyst in the regenerator by air oxidation. Here at least a portion of sulfur, along with carbon and hydrogen, which is deposited on the catalyst, is oxidized and leaves the regenerator in the form of sulfur oxides (SO.sub.2 and SO.sub.3 =SOx) along with substantial amounts of CO, CO.sub.2 and H.sub.2 O. These oxidation reactions are highly exothermic and result in a release of heat in the regenerator. High temperatures can result in structural damage to units not designed to withstand high temperatures. Oxidation is carried out in many FCC units in an oxygen-lean atmosphere at about 1100.degree. to 1250.degree. F., resulting in incomplete combustion of carbon or carbon monoxide to a mixture with a CO.sub.2 /CO ratio, for example, in the range of about 1/1 to 2/1. Some newer FCC units are equipped to operate under conditions such that there is substantially complete combustion of CO in the dense phase of the regenerator. These units operate above about 1250.degree. F.
Considerable recent research effort has been directed to the reduction of sulfur oxide emissions in stack gases from the regenerators of cyclic FCC units. One technique involved circulating one or more metal oxides capable of associating with oxides of sulfur with the cracking catalyst inventory in the regeneration zone. When the particles containing associated oxides of sulfur are circulated to the reducing atmosphere of the cracking zone, the associated sulfur compounds are released as gaseous sulfur-bearing material such as hydrogen sulfide which is discharged with the products from the cracking zone and are in a form readily handled in FCC units. The metal reactant is regenerated to an active form, and is capable of further associating with sulfur oxides when cycled to the regenerator.
Incorporation of Group II metal oxides on particles of cracking catalyst in such a process has been proposed (U.S. Pat. No. 208/120 3,835,031 to Bertolacini). In a related process described in U.S. Pat. No. 208/120 4,071,430 to Blanton et al, discrete fluidizable alumina-containing particles are circulated through the cracking and regenerator zones along with physically separate particles of the active zeolitic cracking catalyst. The alumina particles pick up oxides of sulfur in the regenerator, forming at least one solid compound, including both sulfur and aluminum atoms. The sulfur atoms are released as volatiles, including hydrogen sulfide, in the cracking unit. U.S. Pat. No. 4,071,436 further discloses that 0.1 to 10 weight percent MgO and/or 0.1 to 5 weight percent Cr.sub.2 O.sub.3 are preferably present in the alumina-containing particles. Chromium is used to promote coke burnoff. Similarly, a metallic component, either incorporated into catalyst particles or present on any one of a variety of "inert" supports, is exposed alternately to the oxidizing atmosphere of the regeneration zone of an FCCU and the reducing atmosphere of the cracking zone to reduce sulfur oxide emissions from regenerator gases in accordance with the teachings of Belgian Patents 849,635, 839,636 and 849,637 (1977). In Belgian 849,637, a metallic oxidation promoter such as platinum is also present when carbon monoxide emissions are to be reduced. These patents disclose nineteen different metallic components, including materials as diverse as alkaline earths, sodium, heavy metals and rare earth, as being suitable reactants for reducing emissions of oxides of sulfur. The metallic reactants that are especially preferred are sodium, magnesium, manganese and copper. When used as the carrier for the metallic reactant, the supports that are used preferably have a surface area at least 50 square meters per gram. Examples of allegedly "inert" supports are silica, alumina and silica-alumina. The Belgian patents further disclose that when certain metallic reactants (exemplified by oxides of iron, manganese or cerium) are employed to capture oxides of sulfur, such metallic components can be in the form of a finely divided fluidizable powder.
Similarly, a vast number of sorbents have been proposed for desulfurization of non-FCCU flue gases in zones outside the unit in which SOx is generated. In some such non-FCCU applications, the sorbents are regenerated in environments appreciably richer in hydrogen than the cracking zone of an FCC unit. Cerium oxide is one of fifteen adsorbents disclosed for flue gas desulfurization in a publication of Lowell et al, "SELECTION OF METAL OXIDES FOR REMOVING SOx FROM FLUE GAS," Ind. Eng. Chemical Process Design Development, Vol. 10, No. 3, 1971. In U.S. Pat. No. 4,001,375 to Longo, cerium on an alumina support is used to absorb SO.sub.2 from non-FCCU flue gas streams or automobile exhaust at temperatures of 572.degree. to 1472.degree. F., preferably 932.degree. to 1100.degree. F. The sorbent is then regenerated in a separate unit by contacting it with hydrogen mixed with steam at 932.degree. to 1472.degree. F. During regeneration the desorbed species is initially SO.sub.2 and subsequently H.sub.2 S is evolved. The resulting mixture of SO.sub.2 and H.sub.2 S along with excess reducing gases can be used as feedstock for a Claus unit. The Longo patent is not concerned with reducing emissions from a FCC unit and the reducing atmosphere employed in practice of his process differs significantly from the hydrocarbon-rich atmosphere in a catalytic cracker. Thus a hydrocarbon cracking reaction zone is preferably operated in the substantial absence of added hydrogen while the presence of sweeping amounts of hydrogen gas is essential to the regeneration step in practice of the process of Longo.
D. W. Deberry et al, "RATES OF REACTION OF SO.sub.2 WITH METAL OXIDES," Canadian Journal of Chemical Engineering, 49, 781 (1971) reports that cerium oxide was found to form sulfates more rapidly than most of the other oxides tested. The temperatures used, however, were below 900.degree. F. and thus below those preferred for use in catalyst regenerators in FCC units.
Many commercial zeolitic FCC catalysts contain up to 4% rare earth oxide, the rare earth being used to stabilize the zeolite and provide increased activity. The rare earths are most often used as mixtures of La.sub.2 O.sub.3, CeO.sub.2, Pr.sub.6 O.sub.11, Nd.sub.2 O.sub.3 and others. Some catalyst is produced by using a lanthanum-rich mixture obtained by removing substantial cerium from the mixture of rare earth. It has been found that the mere presence of rare earth in a zeolitic cracking catalyst will not necessarily reduce SOx emissions to an appreciable extent.
In accordance with the teachings of U.S. Pat. No. 3,823,092 to Gladrow, certain zeolitic catalyst compositions capable of being regenerated at a rate appreciably faster than prior art rare earth exchanged zeolitic catalyst compositions are produced by treating a previously rare earth exchanged zeolitic catalyst composition with a dilute solution containing cerium cations ( or a mixture of rare earths rich in cerium). The final catalysts contain 0.5 to 4% cerium cations which are introduced to previously rare earth exchanged zeolitic catalyst particles prior to final filtering, rinsing and calcining. Cerium is described as an "oxidation promoter". There is no recognition or appreciation in the patent of the effect of the cerium impregnation on SOx stack emissions. Such impregnation of rare earth exchanged zeolitic catalyst particles is not always effective in producing modified catalysts having significant ability to bind oxides of sulfur in a FCC regenerator and release them in a FCC cracking reaction zone.
Thus, considerable amount of study and research effort has been directed to reducing oxide of sulfur emissions from various gaseous streams, including those from the stacks of the regenerators of FCC units. However, the results leave much to be desired. Many metallic compounds have been proposed as mat aerials to pick up oxides of sulfur in FCC units (and other desulfurization applications) and a variety of supports, including particles of cracking catalysts and "inerts," have been suggested as carriers for active metallic reactants. Nevertheless, prior to the present invention, a versatile effective technique for using a metallic compound to pick up and then release SOx in a FCC unit without impairing the effectiveness of the active zeolitic cracking catalyst has not met general acceptance in refineries. Many of the proposed metallic reactants lose effectiveness when subjected to repeated cycling. Thus when Group II metal oxides are impregnated on FCC catalysts or various supports, the activity of the Group II metals is rapidly deactivated under the influence of steam. Discrete alumina particles, when combined with silica-containing catalyst particles and subjected to steam at elevated temperatures, e.g., those present in FCC unit regenerators, are of limited effectiveness in reducing SOx emissions. Incorporation of sufficient chromium on an alumina support to improve SOx sorption results in undesirably increased coke and gas production. It has been found that members of the allegedly "inert" supports for metallic reactants mentioned in the Belgian patents (supra) are not capable of stabilizing metallic compounds theoretically capable of picking up SOx in a regenerator and releasing sorbed sulfur in the cracking zone.
Accordingly, an object of the instant invention is the provision of improved means for reducing emissions of sulfur oxides, and optionally carbon monoxide, from FCC units by circulating a reagent for associating with SOx in a regenerator and disassociating sulfur compounds in the cracking unit reaction zone and/or the stripping zone.